1. Field of the Invention
The present invention relates to an exhaust emission control device for an internal combustion engine, and is suitably applied particularly to an exhaust gas purifying system, which employs a selective reduction catalyst (SCR: Selective Catalytic Reduction) for selectively purifying NOx (nitrogen oxide) in exhaust gas by ammonia as a reducing agent. This system is generally known as a urea SCR system because urea water solution is used as a reducing agent.
2. Description of Related Art
Recent years, a urea SCR system is being developed as an exhaust gas purifying system which purifies NOx in exhaust gas at a high purifying rate in an engine (particularly a diesel engine) applied to a vehicle or the like, and some of such a system has been put in practical use. The following configuration is known as a urea SCR system. In the urea SCR system, a selective reduction NOx catalyst is provided in an exhaust pipe connected to an engine main body, and a urea water addition valve for adding urea water as a NOx reducing agent (urea water solution) to the inside of the exhaust pipe is provided on an upstream side of the NOx catalyst.
In the above-described system, by supplying urea water to the inside of the exhaust pipe through the urea water addition valve, NOx in exhaust gas is selectively reduced and removed on the NOx catalyst. In time of the reduction of NOx, ammonia (NH3) is generated through the hydrolyzation of urea water by exhaust heat, and by adsorption of the ammonia to the NOx catalyst and reduction reaction on the NOx catalyst based on ammonia, NOx is reduced and purified.
NO (nitrogen monoxide) and NO2 (nitrogen dioxide) are contained in NOx in exhaust gas. Control of an amount of urea water added and supplied to exhaust gas in accordance with proportion between an amount of NO and an amount of NO2 is proposed. For instance, in an exhaust gas purification unit for diesel engines described in Japanese Unexamined Patent Application Publication No. 2002-250220, proportion of NO/NO2 in exhaust gas is calculated based on outlet temperature of a DPF (Diesel Particulate Filter) as a particulate matter reduction device, and based on the proportion of NO/NO2, an amount of urea supplied to the NOx catalyst is controlled.
According to the invention in the above Publication No. 2002-250220, the amount of urea is controlled in view of components of exhaust gas (proportion of NO/NO2) flowing into the NOx catalyst. In actual NOx purification in the NOx catalyst, it seems that not only the component of inflow gas, but also what kind of NOx reaction is caused on the catalyst needs to be considered. More specifically, chemical reactions differ between NO and NO2 in the NOx catalyst, and thus it may be necessary to consider which component is reduced and purified.
Furthermore, during actual chemical reaction in the NOx catalyst, a chemical reaction which reduces NO and NO2 at the same time (see (formula 1) discussed below) is produced, in addition to a chemical reaction which reduces NO (see (formula 2) discussed below) and a chemical reaction which reduces NO2 (see (formula 3) discussed below). Accordingly, even if the ratio of NO/NO2 of inflow gas is constant, a consumed amount of ammonia (i.e., amount of ammonia which reacts with NOx) may vary with a generation ratio of each chemical reaction or the like. Therefore, a difference of control is caused in an ammonia adsorbed amount and as a result, NOx purification performance may deteriorate.